The quadrupole mass spectrometer is a well-known type of mass spectrometer using a quadrupole mass filter as a mass analyzer for separating ions according to their mass-to-charge ratios. FIG. 11(a) is a schematic diagram showing the configuration of a typical quadrupole mass spectrometer. In this quadrupole mass spectrometer, sample molecules are ionized by an ion source 1, such as an electron-impact ionizer. The ions thus produced are then converged (and accelerated in some cases) by an ion optical system 2, such as an ion lens, and introduced into a space extending along the major axis of a quadrupole mass filter 3 consisting of four rod electrodes. A voltage generated by superposing a direct-current (DC) voltage on a radio-frequency voltage is applied to each of the four electrodes of the quadrupole mass filter 3 so as to select ions in such a manner that an ion having a specific mass-to-charge ratio corresponding to the applied voltages is selectively allowed to pass through the axially extending space while other ions are diverged halfway. The ions that have passed through the quadrupole mass filter 3 are introduced into a detector 4, from which electrical signals corresponding to the amount of the ions are extracted.
The mass-to-charge ratio of the ion that can pass through the quadrupole mass filter 3 basically changes according to the amplitude of the radio-frequency voltage and the DC voltage applied to the filter 3. Therefore, it is possible to scan the mass-to-charge ratio of the ion reaching the detector 4 over a predetermined mass range by scanning the aforementioned voltage values so that they increase or decrease with the lapse of time. This is the scan measurement by the quadrupole mass spectrometer. In addition, an ion-drawing bias voltage, which is a DC voltage, is commonly superposed on the ion-selecting voltages applied to the rod electrodes of the quadruple mass filter 3. This bias voltage creates an appropriate DC electric field in a space between the quadruple mass filter 3 and the ion optical system 2 in order to draw ions from that space into the quadrupole mass filter 3.
The scan speed at which the mass-to-charge ratio is scanned during the scan measurement influences the mass resolution in a mass spectrum or the time resolution of a gas chromatograph/mass spectrometer (GC/MS) or liquid chromatograph/mass spectrometer (LC/MS) in creating a mass chromatogram or total ion chromatogram. Therefore, the scan speed is generally provided as one of the condition parameters that can be set by operators according to the purpose of analysis or the kind of sample. In a conventional quadrupole mass spectrometer, the ion-drawing bias voltage applied to the rod electrodes of the quadrupole mass filter 3 is maintained constant even when the scan speed is changed. This method causes the following problem: Let t denote the time required for an ion to pass through the space (length=L) extending along the major axis of the quadrupole mass filter 3, as shown in FIG. 11(b). This time t depends on the kinetic energy of the ion at a point in time where the ion has reached the inlet of the quadrupole mass filter 3. As explained earlier, during the scan measurement, the ion-selecting voltages applied to the quadrupole mass filter 3 are scanned so that they will continuously change. This voltage change also takes place while the ion is passing through the axially extending space. As the scan speed is raised, the voltage change ΔV during the time t becomes larger.
There will be no practical problem if the scanning time is much longer than the passing time of the ion and the voltage change ΔV is negligibly small. However, if the scan speed is raised (and the scanning time is shortened), the voltage change ΔV that occurs during the passage of the ion through the quadrupole mass filter 3 becomes larger. If the voltage change ΔV is too large to be disregarded, a portion of the ions that should pass through the quadrupole mass filter 3 will be prevented from passing through, so that the quantity of ions reaching the detector 4 will decrease. Thus, the detection sensitivity deteriorates as the scan speed is raised.
In view of such a problem, a mass spectrometer disclosed in Patent Document 1 changes the ion-drawing bias voltage applied to the rod electrodes of the quadrupole mass filter 3 according to the scan speed so that the influence of the change in the scanning voltage during the passage of the ions through the quadrupole mass filter 3 is alleviated. Specifically, when the scan speed of the scan measurement is high, the ion-drawing bias voltage is changed so that ions being introduced into the quadrupole mass filter 3 will have higher levels of kinetic energy. This method avoids the aforementioned decrease in the detection sensitivity even when the scan speed is raised.
Generally, quadrupole mass spectrometers have an auto-tuning mechanism for correcting errors between the mass-to-charge ratio that is intended to be selected by applying a specific ion-selecting voltage to the quadrupole mass filter 3 and the mass-to-charge ratio of the ion that has actually passed through the quadrupole mass filter 3 and reached the detector 4, or for determining optimal voltages to be applied to the ion source 1, the ion optical system 2 and other sections (for example, refer to Patent Document 2). In the auto-tuning mode, an auto-tuning operation is performed using a standard sample prepared for mass calibration. In this operation, a component of the standard sample is mass-analyzed and necessary tuning tasks are performed so that the mass-to-charge ratio corresponding to the aforementioned component comes to a predetermined position in the mass spectrum. In another case, the voltages applied to the relevant sections of the apparatus are adjusted so that the detection signal of the aforementioned component is maximized. Information obtained by such tuning operations is stored in a memory device.
The aforementioned auto-tuning operation is performed before an analysis of an unknown sample, i.e. the target sample. Subsequently, when an operator sets condition parameters, such as the mass range and scan speed, the apparatus selects an appropriate voltage-applying pattern and sets voltages to be applied to the relevant sections on the basis of the information stored in the memory device. Under these conditions, the analysis is performed.
However, the auto-tuning operation performed by the previously described quadrupole mass spectrometer does not include determining an appropriate ion-drawing bias voltage for each scan speed. Therefore, changing the ion-drawing bias voltage according to the scan speed during the actual analysis of an unknown sample does not always guarantee that the DC electric field within the space between the quadrupole mass filter 3 and the ion optical system 2 is optimized in terms of maximization of the detection signal of the objective ion. Accordingly, the detection sensitivity is likely to be sacrificed when the speed of scanning the mass-to-charge ratio is set at high levels.
The technique described in Patent Document 1, in which the ion-drawing bias voltage applied to the rod electrodes of the quadrupole mass filter 3 is changed according to the scan speed, hardly ensures high detection sensitivity over the entire mass range. The reason is as follows: Neglecting the initial energy of the ion, the flight speed v of an ion passing through the quadrupole mass filter 3 is theoretically given by the following equation:(½)mv2=eE  (1),where E is the ion-drawing bias voltage, m is the mass of the ion, and e is the elementary electric charge (1.602×10−19). From this equation:v=(2eE/m)1/2  (2).Accordingly, the time t required for the ion to pass through the quadrupole mass filter 3 having a space length L is given by:t=L/v=L/(2eE/m)1/2=L×(m/2eE)1/2  (3).
The relationship between the scan speed and the time required for measuring one mass unit (which is assumed as “1 m/z” in the present case) is as shown in FIG. 12. For example, when the scan speed is 15000 [amu/sec], the measurement time for one mass unit is 66.67 [μsec]. This means that, if the time required for an ion to pass through the quadrupole mass filter 3 is longer than 66.67 [μsec], the ion cannot reach the detector 4 within the data measurement cycle and causes a decrease in the detection sensitivity. As is clear from equation (2), the ion speed v decreases as the mass m increases. This suggests that, even if the detection sensitivity for ions having relatively small mass-to-charge ratios is adequately high, the detection sensitivity for ions having relatively large mass-to-charge ratios is likely to be low. This deterioration in the detection sensitivity can be avoided by raising the ion-drawing bias voltage so that the ions can more quickly pass through. However in this case, the mass resolution of the resulting mass spectrum may deteriorate due to a decrease in the number of oscillations of the ion or dispersion of kinetic energy of the ion within the quadrupole electric field created by the rod electrodes.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-25498    Patent Document 2: Japanese Patent No. 3478169 (Paragraph [0018])